The present invention relates generally to process fluid or gas bearings, and more particularly provides a novel method of applying a dry lubricant coating to the rubbing contact surface of a foil element used in a high speed gas foil bearing.
Process fluid or gas bearings are now being utilized in an increasing number of diverse applications. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween filled with a fluid such as air, which, under dynamic conditions forms a supporting fluid "wedge" which prevents contact between the two relatively movable elements.
More recently, improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. Such foils, which are generally thin sheets of a compliant material, are deflected by the hydrodynamic film forces between adjacent bearing surfaces and the foils thus enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect.
The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated bearings particularly attractive for high speed rotating machinery. While in many cases the hydrodynamic or self-acting fluid bearings provide sufficient load bearing capacity solely from the pressure generated in the fluid film by the relative motion of the two converging surfaces, it is sometimes necessary to externally pressurize the fluid between the bearing surfaces to increase the load carrying capacity. While these externally pressurized or hydrostatic fluid bearings do increase the load carrying capacity, they do introduce the requirement for an external source of clean fluid under pressure.
Illustrative of hyrodynamic and/or hydrostatic bearing patents assigned to the same Assignee of this application are U.S. Pat. Nos.: 3,215,479; 3,215,480; 3,366,427; 3,375,046; 3,382,014; 3,434,762; 3,467,451; 3,511,544; 3,560,064; 3,615,121; 3,635,534; 3,642,331; 3,677,612 and 3,893,733.
In the operation of these fluid film foil bearings, at startup and rundown and in some cases even at higher speeds, there is actual rubbing contact between the foils and the bearing surfaces with respect to which there is relative movement. This may be between the foils and a shaft or bushing or, in the case of thrust bearings, with respect to a thrust plate or runner. In any case there may also be rubbing contact where individual foils or foil stiffener elements overlap.
In order to lower the startup friction and prevent wear or galling at these contact or rubbing surfaces, wherever they may be, the foils, usually a thin compliant metallic material, are often uniformly coated with a dry lubricating material which is generally softer than the contacting surface. The lubricant material may be of one kind or a mixture of such substances as fluorinated hydrocarbon polymer, graphite, or molybdenum disulfide, all of which are characteristically difficult to make adhere to any metal substrate. Usually they are mixed with a binder to produce better adhesion and other substances to increase their hardness, temperature, and wear resistance. In addition, the foil surface may be etched by various methods such as acid dipping or grit blasting or the coating may be applied by plasma spray or ion deposition means. Sometimes a primer coating with lesser lubricating qualities is applied first. Examples of patents specifically directed to foil coatings are U.S. Pat. Nos. 3,677,612 and 4,005,914 and British Pat. No. 821,954. U.S. Pat. application Ser. No. 33,931, filed Apr. 27, 1979, now abandoned, entitled "Foil Bearing Surface and Method of Making Same" and assigned to the same Assignee as this application is another example.
While these foil bearing rubbing surface coatings have improved bearing wear performance, higher operating temperature requirements now require improved coatings which can survive these higher operating temperatures. Foil element lubricant coatings simply must have higher temperature capabilities if the use of foil bearings is to continue to expand into higher temperature operating environments.
A dry lubricant material which has demonstrated increased operating temperature capabilities in relatively low speed conventional bearing test applications is graphite fluoride (CFx), a solid lubricant which is prepared by the direct chemical combination of graphite powder and fluorine gas under carefully controlled conditions.
Although attempts have been made to apply CFx powder directly to bearing rubbing contact surfaces (i.e., by burnishing techniques), a somewhat better approach has been to mix the powder with a liquid binder such as polyimide resin. The CFx-binder mixture is then applied to the particular bearing surface and cured to form a dry lubricant film on such surface. A conventional method of forming this type of dry lubricant film on a bearing element substrate is set forth in the 1972 National Aeronautics and Space Administration Technical Note NASA TN D-6714 entitled "Graphite Fluoride as a Solid Lubricant in a Polyimide Binder".
Despite the demonstrated high temperature lubricating capabilities of conventionally applied CFx-polyimide binder films in low speed applications, they would be wholly unsatisfactory as lubricant coatings for the compliant foil members in foil bearings which are commonly operated at rotational speeds well in excess of 100,000 RPM. This unsuitability stems from the failure of conventional application techniques to afford the resulting dry lubricant film a satisfactorily uniform thickness and sufficiently smooth rubbing contact surface across the length and breadth of the foil element.
Lacking the requisite high degree of surface smoothness and thickness uniformity, conventionally applied CFx-binder dry lubricant films would exhibit only very limited, and thus unacceptable, life spans in foil bearing applications. Specifically, such films would rather quickly begin to deteriorate if exposed to the high temperature and speed environment characteristic of gas foil bearings.
Accordingly, it is an object of the present invention to eliminate or minimize above-mentioned and other problems by providing an improved method of applying a CFx-binder dry lubricant coating to a gas foil bearing element.